All images shown are based on averaging 3 subsequent excitation pulses at each scan position. Following acquisition of the PA signals, the following protocol was used to reconstruct and display the images: (1) To correct for the effects of photodiode working point the signals were normalised by the power incident on the PD at each scan position which was acquired during the characterisation. (2) For the mouse image the recorded PA signals were interpolated onto a three times finer spatial grid. (3) The tissue sound speed was estimated using an autofocus method [1]}. (4) A three-dimensional image was then reconstructed from the interpolated PA signals using a time-reversal-based algorithm [2]} for the mouse image and a back-projection algorithm [3]} for the wire phantom with the sound speed obtained in step (4) as an input parameter. The image reconstruction was implemented using k-Wave, an open-source Matlab toolbox (www.k-wave.org; [3]}).
